In the related fields of petrochemical industry, liquid phase oxidation reactions of hydrocarbons play significant roles, in particular, the oxidation of cyclohexane. Cyclohexane is oxidized to give oxidation products, such as cyclohexanone, cyclohexanol, hexanedioic acid, and the like. Among them, cyclohexanone and cyclohexanol are useful starting materials, and can be widely applied to fibers, synthetic rubbers, industrial coatings, medicaments, agrochemicals, organic solvents, etc. In addition, cyclohexanone and cyclohexane can further be converted into caprolactam, and then used to prepare polyamides, such as nylons. Therefore, the oxidation reaction of cyclohexane for producing cyclohexanone and cyclohexanol is very valuable from the industrial point of view.
Commercial processes for oxidizing cyclohexane are typically operated at temperatures within the range of 140 to 170° C. under air pressures from 5 to 25 bar or more. In these processes, cyclohexane is oxidized by air in the presence or absence of catalyst to give cyclohexyl hydroperoxide intermediate, and the cyclohexyl hydroperoxide successively undergoes thermal decomposition to form cyclohexanone and cyclohexanol. Because cyclohexanone and cyclohexanol are more oxidizable than cyclohexane, a large quantity of by-products is obtained in the oxidation stage. In order to reduce the proportion of undesired by-products, it is necessary to control the degree of conversion of cyclohexane.
TW Patent 150309 discloses a method for oxidizing cyclohexane in liquid phase. In this method, a high conversion of cyclohexane is achieved in a bubble reactor containing cycloaliphatic acid cobalt salts that act as catalyst under a condition of limited oxygen molecules dissolved in the liquid phase. U.S. Pat. No. 4,341,907 teaches an oxidation reaction of cyclohexane in the presence of a high concentration of cobalt salt catalysts greater than 0.1%. However, the conversion obtained by such a method is not satisfactorily improved. U.S. Pat. No. 4,658,056 discloses using a cobalt compound with ligands that act as first metal catalyst and cobalt naphthenate that act as second metal catalyst to perform the catalytic oxidation reaction of cyclohexane so as to increase the yield of products. But, such a two-catalyst system offers no advantage of reducing cost and simplifying operation.
It is already known the conversion of cyclohexane must be maintained low so as to increase the reaction selectivity for preparing cyclohexanone and cyclohexanol target products. However, a drawback of low conversion is that a large quantity of unreacted cyclohexane is circulated in the operation units, and this causes large energy consumption and can't fulfill economic requirement.
Therefore, it is desired to provide a method for efficiently oxidizing cyclohexane in liquid phase with simple operations and low costs.
To overcome the above-mentioned problems, the present invention has been accomplished after the present inventors made extensive researches and improvements.